1. Technical Field
The present invention relates to a nitride semiconductor laser device applicable to a blue-violet semiconductor laser device used for a light source for writing and reading a high-density optical disk.
2. Related Art
A group III-V nitride compound semiconductor (generally expressed by InGaAlN) typified by gallium nitride (GaN) is a material having a wide forbidden band width (in the case of GaN, 3.4 eV at room temperature) and capable of realizing a light emitting device emitting light in the range of a wavelength from green to ultra-violet. Green/blue light emitting diodes have been commercialized and widely used in various displays and signalers. Moreover, a white light emitting diode that excites a fluorescent material by a blue light emitting diode or an ultra-violet light emitting diode to produce white light has been also commercialized and has been used as a liquid crystal backlight, for example.
It is a blue-violet laser device applicable to a light source for a next-generation high-density optical disk that is expected following the light emitting diodes as a new application field of a nitride compound semiconductor. Through the progression of research and development on epitaxial growth and the process technology of a device, the blue-violet laser device using the nitride compound semiconductor have reached to a level satisfying the specification of a next-generation optical disk typified by Blu-ray. A GaN-based blue-violet semiconductor laser device now reported and commercialized generally uses GaN for a substrate for epitaxial growth (see, for example, S. Nakamura et al., Jpn. J. Appl. Phys., Vol. 37 (1998) L309). This is because a substrate having less crystal defects is desired to improve reliability and because an excellent cleavage face is desired to secure a sufficiently excellent mirror reflectivity to thereby realize a low operating current.
However, a present available GaN substrate is manufactured not by a so-called bulk crystal growing method such as a pulling method but by a hydride vapor phase epitaxy (HVPE) method, so there is a limit in an improvement in throughput and in increasing an area. For this reason, the cost of the substrate is high and hence it is predicted that there will be a limit in reducing the cost of the GaN-based semiconductor laser device. Thus, to put a next-generation optical disk system using the GaN-based semiconductor laser device into widespread use, a reduction in the cost of the laser device is absolutely necessary.
A technology for epitaxial growth on a silicon (Si) substrate has received widespread attention as a technology for realizing a GaN-based device at a lower cost. The crystallinity of the GaN-based semiconductor has been greatly improved by improvements in a buffer layer technology or the like and, for example, a blue light emitting diode formed on a Si substrate has been reported (see, for example, T. Egawa et al., IEEE Electron Device Lett., Vol. 26 (2005), p. 169). If the technology of epitaxial growth on the Si substrate having a large area and produced at a low cost can be applied to a semiconductor laser structure, it is expected that the cost of a blue-violet semiconductor laser device can be drastically reduced.
Moreover, as a semiconductor laser device using a Si substrate is disclosed an example that selectively forms a groove having a V-shaped cross section on a principal surface of the Si substrate, the principal surface having a plane at an off angle of 7.3 degrees with respect to the (100) plane, and that epitaxially grows GaN in which the plane orientation of a principal surface is a (1-101) plane on the principal surface of the Si substrate to thereby construct a semiconductor laser structure (see, for example, Japanese Patent Unexamined Publication No. 2004-031657, hereinafter referred to it as Patent Document 1).
However, the epitaxial growth of the GaN-based semiconductor on the Si substrate in the related art has been entirely performed on a Si substrate in which the plane orientation of a principal surface is a (111) plane. When this is applied to the manufacture of a blue-violet semiconductor laser device, a cleavage face in a laser structure made of the GaN-based semiconductor becomes a (110) plane of Si and a plane equivalent to the (110) plane (for example, a plane equivalent to the (110) plane is hereinafter expressed by a {110} plane), the (110) plane of Si being slanted with respect to both of a (111) plane which is a plane orientation of a principal surface of the Si substrate and a (0001) plane which is a plane orientation of a principal surface of GaN or the like grown on the (111) plane. Thus, there is presented a problem that a cleavage face perpendicular to the principal surface of a laser structure cannot be obtained.
Furthermore, as described in Patent Document 1, when a GaN-based semiconductor is grown on a Si substrate in which the plane orientation of a principal surface is nearly a (100) plane, the Si substrate can be cleaved at a plane equivalent to a (110) plane, but the waveguide of a laser structure described in Patent Document 1 is presumed to have a <11-20> direction. For this reason, the cleavage face of the laser structure in this case is made a (11-20) plane. Thus, when considering the relationship of a crystal orientation between the Si substrate and the GaN-based semiconductor epitaxially grown on the Si substrate, the cleavage face of the laser structure in Patent Document 1 does not match with the cleavage face of the Si substrate, so a good cleavage face cannot be produced. In addition, the description relating to the cleavage face of a laser structure made of a Si substrate and a GaN-based semiconductor is not provided in Patent Document 1.
Hence, when a GaN-based semiconductor laser structure is manufactured by the epitaxial growth method in the related art on a Si substrate having a plane orientation of a (111) plane on a principal surface or on a Si substrate having a plane orientation of a (100) plane on a principal surface and having a groove, which has a V-shaped cross section, selectively formed thereon, a sufficiently large reflectivity cannot be acquired by a facet mirror formed on the cleavage face of a waveguide. As a result, it is difficult to obtain practical values as a threshold current and an operating current.